1. Technical Field of the Invention
This invention relates generally to wireless communication systems and more particularly to transmitters used in such systems.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), radio frequency identification (RFID), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, RFID reader, RFID tag, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system or a particular RF frequency for some systems) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies then. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
As is also known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
Currently, there are two basic types of RF transmitters: Cartesian based transmitter and a Polar coordinate based transmitter. A Cartesian based transmitter includes baseband processing and RF transmission circuitry. The baseband processing encodes, punctures, maps, interleaves, and domain converts outbound data into an in-phase (1) signal component and a quadrature (Q) signal component. For example, if the baseband processing utilizes a 64 quadrature amplitude modulation (QAM) scheme, an a first outbound data value of 101 may be ½ rate encoded into a value of 11 10 01 and a second outbound data value of 011 may be ½ rate encoded into a value of 00 11 01. After puncturing, the encoded values may be interleaved to produce a first interleaved value of 10 11 01 and a second interleaved value of 01 10 01. The first interleaved value is mapped into an I value of 101 and a Q value of 101 and the second interleaved value is mapped into an I value of 011 and a Q value of 001. Each pair of mapped I and Q interleaved values are converted into time domain signals via an inverse fast Fourier transform (IFFT) for a corresponding sub carrier of the signaling protocol (e.g., orthogonal frequency division multiplexing [OFDM]). The time domain I and Q signals are converted into analog signals via an analog to digital converter to produce the I signal component and the Q signal component.
The RF transmission circuitry includes a local oscillator, a mixing section, a linear power amplifier, and may include RF filtering. For direct conversion transmitters, the local oscillator generates an I local oscillation and a Q local oscillation, which are respectively mixed with the I signal component and the Q signal component via the mixing section. The resulting I mixed signal and Q mixed signal are summed to produce an RF signal. The linear power amplifier amplifies to the RF signal to produce an amplified RF signal that may be subsequently bandpass filtered prior to transmission.
While a Cartesian based RF transmitter provides the advantage of a single side band transmitter (i.e., do not have a negative frequencies with I and Q signals), the transmitter path (i.e., the mixing section and the power amplifier) needs to be linear to avoid loss of data resolution. In particular, the linearity requirement limits the output power of the power amplifier.
A Polar coordinate based transmitter includes baseband processing and RF transmission circuitry. The baseband processing encodes, punctures, maps, interleaves, and domain converts outbound data into polar coordinates of an amplitude (A) and a phase (Φ). For example, if the baseband processing utilizes a 64 quadrature amplitude modulation (QAM) scheme, an a first outbound data value of 101 may be ½ rate encoded into a value of 11 11 01 and a second outbound data value of 011 may be ½ rate encoded into a value of 00 11 01. After puncturing, the encoded values may be interleaved to produce a first interleaved value of 10 11 01 and a second interleaved value of 01 10 01. The first interleaved value is mapped into an amplitude value of A0 and a phase value of Φ0 and the second interleaved value is mapped into an amplitude value of A1 and a phase value of Φ1.
The RF transmission circuitry includes a local oscillator and a power amplifier. The local oscillator includes a phase locked loop (PLL) that generates a local oscillation at a desired RF frequency that is modulated based on phase values Φ0 and Φ1. The phase modulated RF signal is then amplitude modulated by the power amplifier in accordance with the amplitude values A0 and A1 to produce a phase and amplitude modulated RF signal.
While the Polar coordinate RF transmitter provides the advantages of reduced RF filtering due to the response of the PLL and the use of a non-linear power amplifier (which, for the same die area, is capable of greater output power than a linear power amplifier), there are some disadvantages. For instance, the response of the PLL is narrow, thus limiting the RF transmitter to narrow band uses. Further, maintaining synchronization between the phase values and the amplitude values can be difficult due to the delays within the PLL. Still further, the baseband processing is utilizing real signals, thus has to account for potential negative frequencies.
From the foregoing, the Cartesian based RF transmitter and Polar based RF transmitter each have their advantages and disadvantages. In addition, both types of transmitters are designed to transmit signals over a wide range of transmit power levels (e.g., from—−50 dB to +28 dB) depending on current transmission conditions. While there is a vast range over which the power amplifier of the transmitter may transmit, empirical data has shown that there is a high probability that, for a majority of the time, the power amplifier will transmit at a power level much smaller than the full range (e.g., −25 dB to +15 dB). In addition, a linear power amplifier is most efficient when operating about 3 dB down from its 1 dB compression point.
Therefore, a need exists for a transmitter and power amplifier module that operates efficiently based on transmit power level probability.